Expand README: comparison table, Mermaid architecture diagram, more examples, badge row

[Copilot]

• Reorganise badge row — PyPI version, Python versions, Coverage prominently first
• Add "⚡ jQuantStats vs QuantStats" comparison table
• Replace ASCII architecture diagram with Mermaid flowchart TD diagram
• Add three new full-code Quick Start examples: risk metrics, benchmark comparison, HTML report
• Fix make validate — corrected examples to use real API (greeks(), to_html()) and assignment style consistent with existing blocks
• Exclude generated report.html from git via .gitignore

Original prompt

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This section details on the original issue you should resolve

<issue_title>Expand README — Add: comparison table vs QuantStats, architecture diagram (Mermaid), more full-code examples, coverage/PyPI/version badges row</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Expand README — Add: comparison table vs QuantStats, architecture diagram (Mermaid), more full-code examples, coverage/PyPI/version badges row #397

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[tschm]

@copilot: fix "make validate"

[Copilot]

@copilot: fix "make validate"

Fixed in 909bb71. The test_readme_runs test in .rhiza/tests/sync/test_readme_validation.py concatenates and executes all Python code blocks in the README. Three issues were found and fixed:

1. data.stats.alpha() / data.stats.beta() don't exist — replaced with data.stats.greeks() which returns {"alpha": ..., "beta": ...} per column
2. pf.report.full() doesn't exist — replaced with pf.report.to_html()
3. print() statements produced unexpected stdout — replaced with variable assignments consistent with the existing README style

Also added report.html to .gitignore since it gets generated when the test runs the code block.